package com.LeetCode.Graph;

import org.junit.Test;

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

/**
 * 拓扑排序
 * 课程表
 */
public class TopologySort {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        int[] indegrees = new int[numCourses];//入度表
        List<List<Integer>> adjacency  = new ArrayList<>();//图
        Queue<Integer> queue = new LinkedList<>();

        for(int i = 0; i<numCourses; i++){
            adjacency.add(new ArrayList<>());
        }

        for(int[] cp : prerequisites){
            indegrees[cp[0]] ++;
            adjacency.get(cp[1]).add(cp[0]);
        }

        for(int i = 0; i<numCourses; i++){
            if(indegrees[i] == 0)//入度为 0 的队列入队
                queue.add(i);
        }

        while(!queue.isEmpty()){
            int pre = queue.poll();
            numCourses--;
            for(int cur : adjacency.get(pre)){
                indegrees[cur]--;
                if(indegrees[cur] == 0) queue.add(cur);
            }
        }

        return numCourses == 0;
    }

    @Test
    public void test(){
        int[][] prerequisites = {{1,0}};
        int num = 2;
        System.out.println(canFinish(num,prerequisites));
    }
}
















